Net Loss per Share - Computation of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator
Net loss									$ (67,698)	$ (57,010)	$ (10,380)
Denominator
Weighted average shares outstanding									38,466,233	33,098,571	28,104,991
Less: weighted average shares subject to repurchase									(79,327)	(266,057)	(629,199)
Weighted average shares used to compute basic and diluted net loss per share	40,259,575	40,116,644	37,440,024	35,827,235	35,684,201	33,525,567	31,200,773	31,089,310	38,386,906	32,832,514	27,475,792
Net loss per share, basic and diluted	$ (0.39)	$ (0.39)	$ (0.49)	$ (0.50)	$ (0.28)	$ (0.50)	$ (0.50)	$ (0.47)	$ (1.76)	$ (1.74)	$ (0.38)